Magnetic resonance imaging (MRI) is a medical imaging modality that can create images of the inside of a human body without using x-rays or other ionizing radiation. MRI uses a powerful magnet to create a strong, uniform, static magnetic field (i.e., the “main magnetic field”). When a human body, or part of a human body, is placed in the main magnetic field, the nuclear spins that are associated with the hydrogen nuclei in tissue water become polarized. This means that the magnetic moments that are associated with these spins become preferentially aligned along the direction of the main magnetic field, resulting in a small net tissue magnetization along that axis (the “z axis,” by convention). A MRI system also comprises components called gradient coils that produce smaller amplitude, spatially varying magnetic fields when current is applied to them. Typically, gradient coils are designed to produce a magnetic field component that is aligned along the z axis and that varies linearly in amplitude with position along one of the x, y or z axes. The effect of a gradient coil is to create a small ramp on the magnetic field strength, and concomitantly on the resonance frequency of the nuclear spins, along a single axis. Three gradient coils with orthogonal axes are used to “spatially encode” the MR signal by creating a signature resonance frequency at each location in the body. Radio frequency (RF) coils are used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei. These coils are used to add energy to the nuclear spin system in a controlled fashion. As the nuclear spins then relax back to their rest energy state, they give up energy in the form of an RF signal. This signal is detected by the MRI system and is transformed into an image using a computer and known reconstruction algorithms.
Magnetic resonance (MR) data may be acquired using an acquisition strategy in which multiple spatial directions are “phase-encoded,” including, for example, three-dimensional (3D) acquisitions and 2D spectroscopic acquisitions. MR data is typically collected in frames that are referred to as “views.” For 3D imaging, each view corresponds to a single ky and kz value, but contains data for the full range of kx values that are required to reconstruct an image. For 2D spectroscopic imaging, each view corresponds to a single kx and ky value, but contains data for the full range of chemical shift frequencies required to reconstruct a spectroscopic image. Many view-ordering techniques are known in the art for determining how ky, kz or kx, ky encoding is performed for each view. View ordering can be an important factor in the quality of the image produced.
Various pulse sequences have periodic signal modulation as a result of, for example, acquiring multiple lines (or “views”) of k-space in a train (or shot) while magnetization is in a transient state or as a result of periodic physiologic motion. Examples of 3D pulse sequences with periodic signal modulation include RARE (Rapid Acquisition with Relaxation Enhancement) sequences (e.g., Fast Spin Echo (FSE) or Turbo Spin Echo (TSE)) that acquire multiple echoes in a train while T2-decay is occurring, fat suppression three-dimensional sequences (such as LAVA and VIBE) that execute multiple repetitions for each fat suppression pulse, inversion recovery gradient echo sequences (such as IR-SPGR (Inversion Recovery SPoiled GRadient echo) and MP-RAGE (Magnetization Prepared Rapid Gradient Echo)) that execute multiple repetitions for each inversion or preparation pulse. Three-dimensional acquisitions may also be segmented to acquire data over multiple cardiac or respiratory cycles. Periodic signal modulation can limit the practical train length (i.e., the number of readouts per train) for the acquisition, the k-space matrix size for the acquisition, and can cause blurring or ringing artifacts. Image artifacts may also be produced as a result of large jumps in k-space between acquired views that can produce erratic phase-behavior.
Various conventional view-ordering techniques have been developed for pulse sequences to produce smooth signal modulation of k-space and provide flexibility in defining train lengths. In one known technique for 3D FSE, the views are ordered such that a kx-ky or kx-kz plane is acquired in an integer number of echo trains. In another known technique for 3D sequences, views from multiple ky-kz planes are acquired in a train by designating separate “turbo factors” for ky and kz. In these techniques, however, sampling is limited to a k-space grid (or matrix) that is regular and rectangular.
To reduce the acquisition time (or scan time) for acquisitions, various methods such as parallel imaging (also known as “partially parallel imaging”) and non-rectangular k-space coverage (e.g., elliptical k-space coverage) may be used. For two-dimensional (2D) accelerated parallel imaging, a non-separable auto-calibration region is most efficient. Non-rectangular k-space coverage also reduces the number of views needed to encode an image dataset. These techniques, however, are not compatible with most current view ordering techniques for sequences with multiple phase encode directions and periodic signal modulation (e.g., 3D FSE).
It would be desirable to provide a method for ordering views for pulse sequences with multiple phase encode directions and periodic signal modulation that improves scanning efficiency, maps signal modulation smoothly into k-space and enables non-rectangular k-space grids and auto-calibrating 2D accelerated parallel imaging. In addition, it would be desirable to provide a view ordering method that orders views so that the steps in k-space from one acquired view to the next are small.